Network Working Group F. Templin, Ed.
Internet-Draft Boeing Research & Technology
Obsoletes: rfc6706 (if approved) July 21, 2014
Intended status: Standards Track
Expires: January 22, 2015
Transmission of IP Packets over AERO Linksdraft-templin-aerolink-30.txt
Abstract
This document specifies the operation of IP over tunnel virtual links
using Asymmetric Extended Route Optimization (AERO). Nodes attached
to AERO links can exchange packets via trusted intermediate routers
that provide forwarding services to reach off-link destinations and
redirection services for route optimization. AERO provides an IPv6
link-local address format known as the AERO address that supports
operation of the IPv6 Neighbor Discovery (ND) protocol and links IPv6
ND to IP forwarding. Admission control and provisioning are
supported by the Dynamic Host Configuration Protocol for IPv6
(DHCPv6), and node mobility is naturally supported through dynamic
neighbor cache updates. Although IPv6 ND messaging is used in the
control plane, both IPv4 and IPv6 are supported in the data plane.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 22, 2015.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
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an IPv6 link-local address constructed as specified in Section 3.2
and applied to a Client's AERO interface.
AERO node
a node that is connected to an AERO link and that participates in
IPv6 ND over the link.
AERO Client ("Client")
a node that applies an AERO address to an AERO interface and
receives an IP prefix delegation.
AERO Server ("Server")
a node that configures an AERO interface to provide default
forwarding services for AERO Clients. The Server applies the IPv6
link-local subnet router anycast address (fe80::) to the AERO
interface and also applies an administratively assigned IPv6 link-
local unicast address used for operation of the IPv6 ND protocol.
AERO Relay ("Relay")
a node that configures an AERO interface to relay IP packets
between nodes on the same AERO link and/or forward IP packets
between the AERO link and the native Internetwork. The Relay
applies an administratively assigned IPv6 link-local unicast
address to the AERO interface the same as for a Server.
ingress tunnel endpoint (ITE)
an AERO interface endpoint that injects tunneled packets into an
AERO link.
egress tunnel endpoint (ETE)
an AERO interface endpoint that receives tunneled packets from an
AERO link.
underlying network
a connected IPv6 or IPv4 network routing region over which the
tunnel virtual overlay is configured.
underlying interface
an AERO node's interface point of attachment to an underlying
network.
link-layer address
an IP address assigned to an AERO node's underlying interface.
When UDP encapsulation is used, the UDP port number is also
considered as part of the link-layer address. Link-layer
addresses are used as the encapsulation header source and
destination addresses.
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network layer address
the source or destination address of the encapsulated IP packet.
end user network (EUN)
an internal virtual or external edge IP network that an AERO
Client connects to the rest of the network via the AERO interface.
AERO Service Prefix (ASP)
an IP prefix associated with the AERO link and from which AERO
Client Prefixes (ACPs) are derived (for example, the IPv6 ACP
2001:db8:1:2::/64 is derived from the IPv6 ASP 2001:db8::/32).
AERO Client Prefix (ACP)
a more-specific IP prefix taken from an ASP and delegated to a
Client.
Throughout the document, the simple terms "Client", "Server" and
"Relay" refer to "AERO Client", "AERO Server" and "AERO Relay",
respectively. Capitalization is used to distinguish these terms from
DHCPv6 client/server/relay. This is an important distinction to
avoid ambiguity, e.g., an AERO Server also acts as a DHCPv6 relay, an
AERO Relay may also act as a DHCPv6 server, etc.
Throughout the document, it is said that an address is "applied" to
an AERO interface since the address need not always be "assigned" to
the interface in the traditional sense. However, the address must at
least be bound to the interface in some fashion for operation of the
IPv6 ND protocol.
The terminology of [RFC4861] (including the names of node variables
and protocol constants) applies to this document. Also throughout
the document, the term "IP" is used to generically refer to either
Internet Protocol version (i.e., IPv4 or IPv6).
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Asymmetric Extended Route Optimization (AERO)
The following sections specify the operation of IP over Asymmetric
Extended Route Optimization (AERO) links:
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In this model, there may be many additional Relays, Servers and
Clients. Each Sever peers with each Relay in a dynamic routing
protocol session to advertise its list of associated Clients. Each
Relay advertises the ASPs for the AERO link into the native IP
Internetwork and serves as a gateway between the AERO link and the
Internetwork. Clients may associate with only a single Server or
with multiple Server, e.g., for fault tolerance and/or load
balancing.
3.2. AERO Node Types
The DHCPv6 server is authoritative for the management of the AERO
link's AERO Service Prefixes (ASPs). The DHCPv6 server is therefore
critical infrastructure for the AERO link, but need not otherwise
participate as an AERO node. AERO Servers communicate with the
DHCPv6 server either via the AERO link itself or via a different IPv6
link.
AERO Relays relay packets between nodes connected to the same AERO
link and also forward packets between the AERO link and the native
Internetwork. The relaying process entails re-encapsulation of IP
packets that were received from a first AERO node and are to be
forwarded without modification to a second AERO node. AERO Relays
present the AERO link to the native Internetwork as a set of one or
more ASPs.
AERO Servers provide default routing services to AERO Clients. AERO
Servers configure a DHCPv6 relay function and facilitate Prefix
Delegation (PD) exchanges between AERO Clients and the DHCPv6 server.
Each delegated prefix becomes an AERO Client Prefix (ACP) taken from
an ASP.
AERO Clients act as requesting routers to receive ACPs through DHCPv6
PD exchanges via AERO Servers over the AERO link. (Each Client MAY
associate with a single Server or with multiple Servers.) Each IPv6
AERO Client receives at least a /64 IPv6 ACP, and may receive even
shorter prefixes. Similarly, each IPv4 AERO Client receives at least
a /32 IPv4 ACP (i.e., a singleton IPv4 address), and may receive even
shorter prefixes.
AERO Clients that act as routers sub-delegate portions of their ACPs
to links on EUNs. End system applications on AERO Clients that act
as routers bind to EUN interfaces (i.e., and not the AERO interface).
AERO Clients that act as ordinary hosts assign one or more IP
addresses from their ACPs to the AERO interface but DO NOT assign the
ACP itself to the AERO interface. Instead, the Client assigns the
ACP to a "black hole" route so that unused portions of the prefix are
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nullified. End system applications on AERO Clients that act as hosts
bind directly to the AERO interface.
3.3. AERO Addresses
An AERO address is an IPv6 link-local address with an embedded ACP
and applied to a Client's AERO interface. The AERO address is formed
as follows:
fe80::[ACP]
For IPv6, the AERO address begins with the prefix fe80::/64 and
includes in its interface identifier the base prefix taken from the
Client's IPv6 ACP. The base prefix is determined by masking the ACP
with the prefix length. For example, if the AERO Client receives the
IPv6 ACP:
2001:db8:1000:2000::/56
it constructs its AERO address as:
fe80::2001:db8:1000:2000
For IPv4, the AERO address is formed as an IPv4-mapped IPv6 address
[RFC4291] that includes the base prefix taken from the Client's IPv4
ACP. For example, if the AERO Client receives the IPv4 ACP:
192.0.2.32/28
it constructs its AERO address as:
fe80::FFFF:192.0.2.32
The AERO address remains stable as the Client moves between
topological locations, i.e., even if its link-layer addresses change.
NOTE: In some cases, prospective neighbors may not have a priori
knowledge of the Client's ACP length and may therefore send initial
IPv6 ND messages with an AERO destination address that matches the
ACP but does not correspond to the base prefix. In that case, the
Client MUST accept the address as equivalent to the base address, but
then use the base address as the source address of any IPv6 ND
message replies. For example, if the Client receives the IPv6 ACP
2001:db8:1000:2000::/56 then subsequently receives an IPv6 ND message
with destination address fe80::2001:db8:1000:2001, it accepts the
message but uses fe80::2001:db8:1000:2000 as the source address of
any IPv6 ND replies.
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Preference is an integer value between 0 and 255 indicating the
node's preference for this underlying interface, with 0 being highest
preference and 255 being lowest. UDP Port Number and IP Address are
set to the addresses used by the target node when it sends
encapsulated packets over the underlying interface. When no UDP
encapsulation is used, UDP Port Number is set to 0. When the
encapsulation IP address family is IPv4, IP Address is formed as an
IPv4-mapped IPv6 address [RFC4291].
When a Relay enables an AERO interface, it applies an
administratively assigned link-local address fe80::ID to the
interface for communicating with Servers on the link. Each fe80::ID
address MUST be unique among all Relays and Servers on the link, and
MUST NOT collide with any potential AERO addresses, e.g., the
addresses could be assigned as fe80::1, fe80::2, fe80::3, etc. The
Relay also maintains an IP forwarding table entry for each Client-
Server association and maintains a neighbor cache entry for each
Server on the link. Relays do not require the use of IPv6 ND
messaging for reachability determination since Relays and Servers
engage in a dynamic routing protocol over the AERO interface. At a
minimum, however, Relays respond to NS messages by returning an NA.
When a Server enables an AERO interface, it applies the address
fe80:: to the interface as a link-local Subnet Router Anycast
address, and also applies an administratively assigned link-local
address fe80::ID to support the operation of the IPv6 ND protocol and
to communicate with Relays on the link. The Server maintains a
neighbor cache entry for each Relay on the link, and also creates
per-Client neighbor cache entries whenever it discovers a new Client.
At a minimum, when the Server receives an NS/RS messages on the AERO
interface it returns an NA/RA message. When the Server receives an
NS/NA, it also update timers in existing neighbor cache entries but
does not create new neighbor cache entries nor update cached link-
layer addresses. Servers also engage in a dynamic routing protocol
with all Relays on the link. Finally, the Server provides a simple
conduit between Clients and Relays, or between Clients and other
Clients. Therefore, packets enter the Server's AERO interface from
the link layer and are forwarded back out the link layer without ever
leaving the AERO interface and therefore without ever disturbing the
network layer.
When a Client enables an AERO interface, it invokes prefix delegation
to receive an ACP. Next, it applies the corresponding AERO address
to the AERO interface, i.e., the prefix delegation bootstraps the
provisioning of a unique link-local address. The Client maintains a
neighbor cache entry for each of its Servers and each of its active
peer Clients. When the Client receives Redirect/Predirect messages
on the AERO interface it updates or creates neighbor cache entries,
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including link-layer address information. Unsolicited NA messages
update the cached link-layer address for the neighbor Client (e.g.,
following a link-layer address change due to node mobility) but do
not create new neighbor cache entries. RA messages as well as NS/NA
messages used for Neighbor Unreachability Detection (NUD) update
timers in existing neighbor cache entires but do not update link-
layer addresses nor create new neighbor cache entries. Redirect,
Predirect and unsolicited NA messages SHOULD include a Timestamp
option (see Section 5.3 of [RFC3971]) that other AERO nodes can use
to verify the message time of origin. Predirect, NS and RS messages
SHOULD include a Nonce option (see Section 5.3 of [RFC3971]) that
recipients echo back in corresponding responses. Finally, the Client
need not maintain any IP forwarding table entries for neighboring
Clients. Instead, it can set a single "route-to-interface" default
route in the IP forwarding table pointing to the AERO interface, and
all forwarding decisions can be made within the AERO interface based
on neighbor cache entries.
3.4.1. Coordination of Multiple Underlying Interfaces
AERO interfaces may be configured over multiple underlying
interfaces. For example, common mobile handheld devices have both
wireless local area network ("WLAN") and cellular wireless links.
These links are typically used "one at a time" with low-cost WLAN
preferred and highly-available cellular wireless as a standby. In a
more complex example, aircraft frequently have many wireless data
link types (e.g. satellite-based, terrestrial, air-to-air
directional, etc.) with diverse performance and cost properties.
If a Client's multiple underlying interfaces are used "one at a time"
(i.e., all other interfaces are in standby mode while one interface
is active), then Redirect, Predirect and unsolicited NA messages
include only a single TLLAO with Link ID set to a constant value
(e.g., 0).
If the Client has multiple active underlying interfaces, then from
the perspective of IPv6 ND it would appear to have a single link-
local address with multiple link-layer addresses. In that case,
Redirect, Predirect and unsolicited NA messages MAY include multiple
TLLAOs -- each with a different Link ID that corresponds to a
specific underlying interface of the Client. Further details on
coordination of multiple active underlying interfaces are outside the
scope of this specification.
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Internet-Draft AERO July 20143.5. AERO Interface Neighbor Cache Maintenace
Each AERO interface maintains a conceptual neighbor cache that
includes an entry for each neighbor it communicates with on the AERO
link, the same as for any IPv6 interface [RFC4861]. Neighbor cache
entries are created and maintained as follows:
AERO Relays maintain a permanent neighbor cache entry for each Server
on the link, and AERO Servers maintain a permanent neighbor cache
entry for each Relay on the link. AERO Clients maintain a neighbor
cache entry for each of their associated Servers, and AERO Servers
maintain a neighbor cache for each of their associated Clients with a
lifetime based on the DHCPv6 lease lifetime. AERO Clients maintain
neighbor cache entries for each of their active correspondent Clients
with lifetimes based on IPv6 ND messaging constants.
When an AERO Server relays a DHCPv6 Reply message to an AERO Client,
it creates or updates a neighbor cache entry for the Client based on
the AERO address corresponding to the Client's ACP as the network-
layer address and with the Client's encapsulation IP address and UDP
port number as the link-layer address. The Server also records the
ACP's lease lifetime and prefix length in the neighbor cache entry.
When an AERO Client receives a DHCPv6 Reply message from an AERO
Server, it creates or updates a neighbor cache entry for the Server
based on the Reply message link-local source address as the network-
layer address, the lease lifetime as the neighbor cache entry
lifetime, and the encapsulation IP source address and UDP source port
number as the link-layer address.
When an AERO Client receives a valid Predirect message it creates or
updates a neighbor cache entry for the Predirect target network-layer
and link-layer addresses plus prefix length. The node then sets an
"AcceptTime" variable for the neighbor and uses this value to
determine whether packets received from the predirected neighbor can
be accepted.
When an AERO Client receives a valid Redirect message it creates or
updates a neighbor cache entry for the Redirect target network-layer
and link-layer addresses plus prefix length. The node then sets a
"ForwardTime" variable for the neighbor and uses this value to
determine whether packets can be sent directly to the redirected
neighbor. The node also maintains a "Retry" variable to limit the
number of keepalives sent when a neighbor may have gone unreachable.
When an AERO Client receives a valid NS message corresponding to a
neighbor cache entry for another Client, it (re)sets AcceptTime for
the neighbor to ACCEPT_TIME.
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When an AERO Client receives a valid solicited NA message
corresponding to a neighbor cache entry for another Client, it
(re)sets ForwardTime for the neighbor to FORWARD_TIME and sets Retry
to MAX_RETRY. (When an AERO Client receives a valid unsolicited NA
message, it updates the neighbor's link-layer address but DOES NOT
reset ForwardTime or Retries.)
It is RECOMMENDED that FORWARD_TIME be set to the default constant
value 30 seconds to match the default REACHABLE_TIME value specified
for IPv6 ND [RFC4861].
It is RECOMMENDED that ACCEPT_TIME be set to the default constant
value 40 seconds to allow a 10 second window so that the AERO
redirection procedure can converge before AcceptTime decrements below
FORWARD_TIME.
It is RECOMMENDED that MAX_RETRY be set to 3 the same as described
for IPv6 ND address resolution in Section 7.3.3 of [RFC4861].
Different values for FORWARD_TIME, ACCEPT_TIME, and MAX_RETRY MAY be
administratively set, if necessary, to better match the AERO link's
performance characteristics; however, if different values are chosen,
all nodes on the link MUST consistently configure the same values.
Most importantly, ACCEPT_TIME SHOULD be set to a value that is
sufficiently longer than FORWARD_TIME to allow the AERO redirection
procedure to converge.
For AERO Client<->Server neighbor cache entries, AcceptTime and
ForwardTime are set based on the DHCPv6 lease lifetime and may be
modified based on the Router Lifetime advertised in the Server's RA
messages.
3.6. AERO Interface Sending Algorithm
When an IP packet enters a Client's AERO interface from the network
layer, the Client searches its neighbor cache for an entry with an
AERO address that matches the packet's destination address. If there
is a match, the Client uses the link-layer address in the neighbor
cache entry as the link-layer address for encapsulation then admits
the packet into the tunnel. If there is no match, the Client instead
uses the link-layer address of a neighboring Server as the link-layer
address for encapsulation. (Note that the Client caches the ASPs for
the AERO link and can thus search the neighbor cache only for
destination addresses that are covered by an ASP.)
When an IP packet enters a Server's AERO interface from the link
layer, the Server searches for a neighbor cache match the same as for
a Client. If there is a match, the Server uses the link-layer
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address in the neighbor cache entry as the link-layer address for re-
encapsulation. If there is no match, the Server instead uses the
link-layer address of a neighboring Relay as the link-layer address
for encapsulation. Servers also relay Predirect, Redirect and
unsolicited Neighbor Advertisement messages received from a Client
and with an AERO destination address. If the AERO destination
address is the address of a neighbor, the Server changes the link-
layer source address to its own address, changes the link-layer
destination address to the address of the neighbor and forwards the
message to the neighbor. If the AERO destination address is not a
neighbor, the Server instead forwards the message to a Relay. When
an AERO Relay forwards either a data packet or an IPv6 ND message to
an AERO Server, the Server MUST NOT forward the packet back to the
same or a different Relay.
When an IP packet enters a Relay's AERO interface from the network
layer, the Relay searches its IP forwarding table for an entry that
is covered by an ASP and also matches the destination. If there is a
match, the Relay uses the link-layer address in the neighbor cache
entry for the next-hop Server as the link-layer address for
encapsulation. When an IP packet enters a Relay's AERO interface
from the link-layer, if the destination is not covered by an ASP the
Relay forwards the packet to another IP link as indicated by the IP
forwarding table. If the destination is covered by an ASP, and there
is a more-specific forwarding table entry that matches the
destination, the Relay uses the link-layer address in the neighbor
cache entry for the next-hop Server as the link-layer address for
encapsulation. If there is no more-specific entry, the Relay instead
drops the packet. Relays also relay Predirect, Redirect and
unsolicited Neighbor Advertisement messages by searching for an IP
forwarding table entry that matches the message's AERO destination
address. If there is a match, the Relay proxies the packet in the
same manner as described for Servers above; otherwise, the Relay
drops the packet. When an AERO Server forwards either a data packet
or an IPv6 ND message to an AERO Relay, the Relay MUST NOT forward
the packet back to the same Server.
Note that in the above this tunnel exit determination is often based
on consulting the neighbor cache instead of the IP forwarding table.
IP forwarding is therefore linked to IPv6 ND via the AERO address.
When an AERO node forwards a packet back out the same AERO interface
the packet arrived on, the node MUST NOT decrement the network layer
TTL/Hop-count.
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Internet-Draft AERO July 20143.7. AERO Interface Encapsulation, Re-encapsulation and Decapsulation
AERO interfaces encapsulate IP packets according to whether they are
entering the AERO interface from the network layer or if they are
being forwarded out the same AERO interface that they arrived on.
This latter form of encapsulation is known as "re-encapsulation".
AERO interfaces encapsulate packets per the specifications in
[RFC2003][RFC2473][RFC4213][RFC4301][RFC5246] (etc.) except that the
interface copies the "TTL/Hop Limit", "Type of Service/Traffic Class"
and "Congestion Experienced" values in the packet's IP header into
the corresponding fields in the encapsulation header. For packets
undergoing re-encapsulation, the AERO interface instead copies the
"TTL/Hop Limit", "Type of Service/Traffic Class" and "Congestion
Experienced" values in the original encapsulation header into the
corresponding fields in the new encapsulation header (i.e., the
values are transferred between encapsulation headers and *not* copied
from the encapsulated packet's network-layer header).
When AERO UDP encapsulation is used, the AERO interface encapsulates
the packet per the specifications in [RFC2003][RFC2473][RFC4213]
except that it inserts a UDP header between the encapsulation header
and the packet's IP header. The AERO interface sets the UDP source
port to a constant value that it will use in each successive packet
it sends, sets the UDP checksum field to zero (see:
[RFC6935][RFC6936]) and sets the UDP length field to the length of
the IP packet plus 8 bytes for the UDP header itself. For packets
sent via a Server, the AERO interface sets the UDP destination port
to 8060 (i.e., the IANA-registered port number for AERO) when AERO-
only encapsulation is used. For packets sent to a neighboring
Client, the AERO interface sets the UDP destination port to the port
value stored in the neighbor cache entry for this neighbor.
The AERO interface next sets the IP protocol number in the
encapsulation header to the appropriate value for the first protocol
layer within the encapsulation (e.g., IPv4, IPv6, UDP, IPsec, etc.).
When IPv6 is used as the encapsulation protocol, the interface then
sets the flow label value in the encapsulation header the same as
described in [RFC6438]. When IPv4 is used as the encapsulation
protocol, the AERO interface sets the DF bit as discussed in
Section 3.7.
AERO interfaces decapsulate packets destined either to the node
itself or to a destination reached via an interface other than the
AERO interface the packet was received on. When AERO UDP
encapsulation is used (i.e., when a UDP header with destination port
8060 is present) the interface examines the first octet of the
encapsulated packet. If the most significant four bits of the first
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octet encode the value '0110' (i.e., the version number value for
IPv6) or the value '0100' (i.e., the version number value for IPv4),
the packet is accepted and the encapsulating UDP header is discarded;
otherwise, the packet is discarded.
Further decapsulation then proceeds according to the appropriate
tunnel type [RFC2003][RFC2473][RFC4213][RFC4301][RFC5246] (etc.).
3.8. AERO Interface Data Origin Authentication
AERO nodes employ simple data origin authentication procedures for
encapsulated packets they receive from other nodes. In particular,
AERO Clients accept encapsulated packets with a link-layer source
address belonging to one of their current AERO Servers, and AERO
Clients and Servers accept encapsulated packets with a link-layer
source address that is correct for the network-layer source address.
The AERO node considers the link-layer source address correct for the
network-layer source address if there is an AERO interface neighbor
cache entry with an AERO address that matches the packet's network-
layer source address prefix, with a link-layer address that matches
the packet's link-layer source address, and AcceptTime is non-zero.
An AERO Server also accepts packets with a link-layer source address
that matches one of its associated Relays, and an AERO Relay accepts
packets with a source address that matches one of its associated
Servers.
Note that this simple data origin authentication only applies to
environments in which link-layer addresses cannot be spoofed.
Additional security mitigations may be necessary in other
environments.
3.9. AERO Interface MTU Considerations
The AERO link Maximum Transmission Unit (MTU) is 64KB minus the
encapsulation overhead for IPv4 as the link-layer [RFC0791] and 4GB
minus the encapsulation overhead for IPv6 as the link layer
[RFC2675]. This is the most that IPv4 and IPv6 (respectively) can
convey within the constraints of protocol constants, but actual sizes
available for tunneling will frequently be much smaller.
The base tunneling specifications for IPv4 and IPv6 typically set a
static MTU on the tunnel interface to 1500 bytes minus the
encapsulation overhead or smaller still if the tunnel is likely to
incur additional encapsulations on the path. This can result in path
MTU related black holes when packets that are too large to be
accommodated over the AERO link are dropped, but the resulting ICMP
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Packet Too Big (PTB) messages are lost on the return path. As a
result, AERO nodes use the following MTU mitigations to accommodate
larger packets.
AERO nodes set their AERO interface MTU to the larger of the
underlying interface MTU minus the encapsulation overhead, and 1500
bytes. (If there are multiple underlying interfaces, the node sets
the AERO interface MTU according to the largest underlying interface
MTU, or 64KB /4G minus the encapsulation overhead if the largest MTU
cannot be determined.) AERO nodes optionally cache other per-
neighbor MTU values in the underlying IP path MTU discovery cache
initialized to the underlying interface MTU.
AERO nodes admit packets that are no larger than 1280 bytes minus the
encapsulation overhead (*) as well as packets that are larger than
1500 bytes into the tunnel without fragmentation, i.e., as long as
they are no larger than the AERO interface MTU before encapsulation
and also no larger than the cached per-neighbor MTU following
encapsulation. For IPv4, the node sets the "Don't Fragment" (DF) bit
to 0 for packets no larger than 1280 bytes minus the encapsulation
overhead (*) and sets the DF bit to 1 for packets larger than 1500
bytes. If a large packet is lost in the path, the node may
optionally cache the MTU reported in the resulting PTB message or may
ignore the message, e.g., if there is a possibility that the message
is spurious.
For packets destined to an AERO node that are larger than 1280 bytes
minus the encapsulation overhead (*) but no larger than 1500 bytes,
the node uses IP fragmentation to fragment the encapsulated packet
into two pieces (where the first fragment contains 1024 bytes of the
original IP packet) then admits the fragments into the tunnel. If
the link-layer protocol is IPv4, the node admits each fragment into
the tunnel with DF set to 0 and subject to rate limiting to avoid
reassembly errors [RFC4963][RFC6864]. For both IPv4 and IPv6, the
node also sends a 1500 byte probe message (**) to the neighbor,
subject to rate limiting.
To construct a probe, the node prepares an NS message with a Nonce
option plus trailing padding octets added to a length of 1500 bytes
without including the length of the padding in the IPv6 Payload
Length field. The node then encapsulates the NS in the encapsulation
headers (while including the length of the padding in the
encapsulation header length fields), sets DF to 1 (for IPv4) and
sends the padded NS message to the neighbor. If the neighbor returns
an NA message with a correct Nonce value, the node may then send
whole packets within this size range and (for IPv4) relax the rate
limiting requirement. (Note that the trailing padding SHOULD NOT be
included within the Nonce option itself but rather as padding beyond
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the last option in the NS message; otherwise, the (large) Nonce
option would be echoed back in the solicited NA message and may be
lost at a link with a small MTU along the reverse path.)
AERO nodes MUST be capable of reassembling packets up to 1500 bytes
plus the encapsulation overhead length. It is therefore RECOMMENDED
that AERO nodes be capable of reassembling at least 2KB.
(*) Note that if it is known without probing that the minimum Path
MTU to an AERO node is MINMTU bytes (where 1280 < MINMTU < 1500) then
MINMTU can be used instead of 1280 in the fragmentation threshold
considerations listed above.
(**) It is RECOMMENDED that no probes smaller than 1500 bytes be used
for MTU probing purposes, since smaller probes may be fragmented if
there is a nested tunnel somewhere on the path to the neighbor.
Probe sizes larger than 1500 bytes MAY be used, but may be
unnecessary since original sources are expected to implement
[RFC4821] when sending large packets.
3.10. AERO Router Discovery, Prefix Delegation and Address Configuration3.10.1. AERO Client Behavior
AERO Clients discover the link-layer addresses of AERO Servers via
static configuration, or through an automated means such as DNS name
resolution. In the absence of other information, the Client resolves
the Fully-Qualified Domain Name (FQDN) "linkupnetworks.[domainname]"
where "[domainname]" is the connection-specific DNS suffix for the
Client's underlying network connection. After discovering the link-
layer addresses, the Client associates with one or more of the
corresponding Servers.
To associate with a Server, the Client acts as a requesting router to
request an ACP through DHCPv6 PD [RFC3315][RFC3633][RFC6355] using
'fe80::ffff:ffff:ffff:ffff' as the IPv6 source address,
'All_DHCP_Relay_Agents_and_Servers' as the IPv6 destination address
and the link-layer address of the Server as the link-layer
destination address. The Client includes a DHCPv6 Unique Identifier
(DUID) in the Client Identifier option of its DHCPv6 messages (as
well as a DHCPv6 authentication option if necessary) to identify
itself to the DHCPv6 server. The Client also includes a DHCPv6
Client Link Layer Address Option (CLLAO) [RFC6939] with the link-
layer address format shown in Figure 2 with Link ID followed by
Preference followed by the values 0 for both the UDP Port Number and
IP Address.The Client sets the CLLAO 'option-length' field to 22 (2
plus the length of the link-layer address) and sets the 'link-layer
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type' field to TBD1 (see: IANA Considerations). If the Client is
pre-provisioned with an ACP associated with the AERO service, it MAY
also include the ACP in its DHCPv6 PD Request to indicate its
preferred ACP to the DHCPv6 server. The Client then sends the
encapsulated DHCPv6 request via an underlying interface.
When the Client receives its ACP and the set of ASPs via a Reply from
the DHCPv6 server, i.e., via an AERO Server acting as a DHCPv6 relay,
it creates a neighbor cache entry with the Server's link-local
address (i.e., fe80::ID) as the network-layer address and the
Server's encapsulation address as the link-layer address.
The Client then applies the AERO address to the AERO interface and
sub-delegates the ACP to nodes and links within its attached EUNs
(the AERO address thereafter remains stable as the Client moves).
The Client also assigns a default IP route to the AERO interface as a
route-to-interface, i.e., with no explicit next-hop.
The Client subsequently renews its ACP delegation by performing
DHCPv6 Renew/Reply exchanges with its AERO address as the IPv6 source
address, 'All_DHCP_Relay_Agents_and_Servers' as the IPv6 destination
address, the link-layer address of a Server as the link-layer
destination address and the same DUID and authentication information.
If the Client wishes to associate with multiple Servers, it can
perform DHCPv6 Renew/Reply exchanges via each of the Servers.
The Client then sends an RS message to each of its associated Servers
to receive an RA message with a Router Lifetime and any other link-
specific parameters. The Client uses the Router Lifetime to set the
lifetime for the neighbor cache entry for this Server. The Client
further ignores any RS messages it might receive, since only Servers
may process RS messages.
The Client then sends periodic RS messages to obtain new RA messages,
and further initiates a new DHCPv6 Renew/Reply exchange before the
Router Lifetime expires. The Client can also forward IP packets
destined to networks beyond its local EUNs via a Server as a default
router.
Since the Client's AERO address is configured from the unique ACP
delegation it receives, there is no need for Duplicate Address
Detection (DAD) on AERO links. Other nodes maliciously attempting to
hijack an authorized Client's AERO address will be denied access to
the network by the DHCPv6 server due to an unacceptable link-layer
address and/or security parameters (see: Security Considerations).
AERO Clients ignore the IP address and UDP port number in any S/TLLAO
options in ND messages they receive directly from another AERO
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Client, but examine the Link ID and Preference values to match the
message with the correct link-layer address information.
When a source Client forwards a packet to a prospective destination
Client (i.e., one for which the packet's destination address is
covered by an ASP), the source Client initiates an AERO route
optimization procedure as specified in Section 3.12.
3.10.2. AERO Server Behavior
AERO Servers configure a DHCPv6 relay function on their AERO links.
AERO Servers arrange to add their encapsulation layer IP addresses
(i.e., their link-layer addresses) to the DNS resource records for
the FQDN "linkupnetworks.[domainname]" before entering service.
Here, "linkupnetworks" is a constant text string, and "[domainname]"
is the connection-specific DNS suffix for this underlying network
connection.
When an AERO Server relays a prospective Client's DHCPv6 PD messages
to the DHCPv6 server, it wraps each message in a "Relay-forward"
message per [RFC3315] and includes a DHCPv6 Interface Identifier
option that encodes a value that identifies the AERO link to the
DHCPv6 server. Without creating internal state, the Server then
modifies the Client's link-layer address in the CLLAO [RFC6939] by
writing the client's UDP Port number and IP adddress in the
corresponding fields of the option. The Server finally includes a
DHCPv6 Echo Request Option (ERO) [RFC4994] that encodes the option
code for the CLLAO in a 'requested-option-code-n' field, then relays
the message to the DHCPv6 server. The CLLAO information will
therefore subsequently be echoed back in the DHCPv6 server's "Relay-
reply" message.
When the DHCPv6 server issues the ASPs and ACP prefix delegation in a
"Relay-reply" message via the AERO Server (acting as a DHCPv6 relay),
the Server obtains the Client's link-layer address from the echoed
CLLAO option and also obtains the Client's delegated ACP and lease
lifetime from the message. The Server then creates a neighbor cache
entry for the Client's AERO address with the Client's link-layer
address as the link-layer address and with lifetime set to no more
than the lease lifetime. The Server finally injects the ACP as an IP
route into the inter-Server/Relay routing system (see: Section 3.11)
then relays the DHCPv6 message to the Client while using fe80::ID as
the IPv6 source address, the link-local address found in the "peer
address" field of the Relay-reply message as the IPv6 destination
address, and the Client's link-layer address as the destination link-
layer address.
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Servers respond to NS/RS messages from Clients on their AERO
interfaces by returning an NA/RA message. When the Server returns an
RA message, it sets Router Lifetime to the neighbor cache entry
lifetime but does not include any Prefix Information Options (PIOs)
since the AERO link is link-local-only. The server decrements the
neighbor cache entry lifetime according to the system clock.
Servers ignore any RA messages they may receive from a Client, but
they MAY examine RA messages received from other Servers for
consistency verification purposes.
3.10.3. DHCPv6 Server Behavior
The DHCPv6 server observes both the base DHCPv6 specification
[RFC3315] and the DHCPv6 PD specification [RFC3633]. The DHCPv6
server further MUST honor the DHCPv6 Echo Request Option (ERO) and
Client Link-Layer Address Option (CLLAO) as discussed in
Section 3.10.1.
The DHCPv6 server also includes a DHCPv6 Vendor-Specific Information
Option with 'enterprise-number' set to "TBD2" (see: IANA
Considerations). The option is formatted as shown in[RFC3315] and
with the AERO enterprise-specific format shown in Figure 3:
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Internetwork as a small set of ASPs as opposed to the full set of
individual ACPs.
In a reference BGP arrangement, each AERO Server is configured as an
Autonomous System Border Router (ASBR) for a stub Autonomous System
(AS) (possibly using a private AS Number (ASN) [RFC1930]), and each
Server further peers with each Relay but does not peer with other
Servers. Similarly, Relays need not peer with each other, since they
will receive all updates from all Servers and will therefore have a
consistent view of the AERO link ACP delegations.
Each Server maintains a working set of associated Clients, and
dynamically announces new ACPs and withdraws departed ACPs in its BGP
updates to Relays. Relays do not send BGP updates to Servers,
however, such that the BGP route reporting is unidirectional from the
Servers to the Relays.
The Relays therefore discover the full topology of the AERO link in
terms of the working set of ACPs associated with each Server, while
the Servers only discover the ACPs of their associated Clients.
Since Clients are expected to remain associated with their current
set of Servers for extended timeframes, the amount of BGP control
messaging between Servers and Relays should be minimal. However, BGP
peers SHOULD dampen any route oscillations caused by impatient
Clients that repeatedly associate and disassociate with Servers.
3.12. AERO Redirection3.12.1. Reference Operational Scenario
Figure 4 depicts the AERO redirection reference operational scenario,
using IPv6 addressing as the example (while not shown, a
corresponding example for IPv4 addressing can be easily constructed).
The figure shows an AERO Relay ('R'), two AERO Servers ('S1', 'S2'),
two AERO Clients ('A', 'B') and two ordinary IPv6 hosts ('C', 'D'):
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EUNs. IPv6 host ('D') connects to the EUN, and configures the
address 2001:db8:1::1.
3.12.2. Concept of Operations
Again, with reference to Figure 4, when source host ('C') sends a
packet to destination host ('D'), the packet is first forwarded over
the source host's attached EUN to Client ('A'). Client ('A') then
forwards the packet via its AERO interface to Server ('S1') and also
sends a Predirect message toward Client ('B') via Server ('S1').
Server ('S1') then re-encapsulates and forwards both the packet and
the Predirect message out the same AERO interface toward Client ('B')
via Relay ('R').
When Relay ('R') receives the packet and Predirect message, it
consults its forwarding table to discover Server ('S2') as the next
hop toward Client ('B'). Relay ('R') then forwards both messages to
Server ('S2'), which then forwards them to Client ('B').
After Client ('B') receives the Predirect message, it process the
message and returns a Redirect message toward Client ('A') via Server
('S2'). During the process, Client ('B') also creates or updates a
neighbor cache entry for Client ('A').
When Server ('S2') receives the Redirect message, it re-encapsulates
the message and forwards it on to Relay ('R'), which forwards the
message on to Server ('S1') which forwards the message on to Client
('A'). After Client ('A') receives the Redirect message, it
processes the message and creates or updates a neighbor cache entry
for Client ('C').
Following the above Predirect/Redirect message exchange, forwarding
of packets from Client ('A') to Client ('B') without involving any
intermediate nodes is enabled. The mechanisms that support this
exchange are specified in the following sections.
3.12.3. Message Format
AERO Redirect/Predirect messages use the same format as for ICMPv6
Redirect messages depicted in Section 4.5 of [RFC4861], but also
include a new "Prefix Length" field taken from the low-order 8 bits
of the Redirect message Reserved field. (For IPv6, valid values for
the Prefix Length field are 0 through 64; for IPv4, valid values are
0 through 32.) The Redirect/Predirect messages are formatted as
shown in Figure 5:
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o the network-layer source address is set to fe80::2001:db8:0:0
(i.e., the AERO address of Client ('A')).
o the network-layer destination address is set to fe80::2001:db8:1:0
(i.e., the AERO address of Client ('B')).
o the Type is set to 137.
o the Code is set to 1 to indicate "Predirect".
o the Prefix Length is set to the length of the prefix to be applied
to the Target Address.
o the Target Address is set to fe80::2001:db8:0:0 (i.e., the AERO
address of Client ('A')).
o the Destination Address is set to the source address of the
originating packet that triggered the Predirection event. (If the
originating packet is an IPv4 packet, the address is constructed
in IPv4-compatible IPv6 address format).
o the message includes a TLLAO with Link ID and Preference set to
appropriate values for Client ('A')'s underlying interface, and
with UDP Port Number and IP Address set to 0'.
o the message SHOULD include a Timestamp option.
o the message includes a Redirected Header Option (RHO) that
contains the originating packet truncated to ensure that at least
the network-layer header is included but the size of the message
does not exceed 1280 bytes.
Note that the act of sending Predirect messages is cited as "MAY",
since Client ('A') may have advanced knowledge that the direct path
to Client ('B') would be unusable. If the direct path later becomes
unusable after the initial route optimization, Client ('A') simply
allows packets to again flow through Server ('S1').
3.12.5. Re-encapsulating and Relaying Predirects
When Server ('S1') receives a Predirect message from Client ('A'), it
first verifies that the requested redirection is authorized. If the
redirection is not permitted, Server ('S1') discards the message.
Otherwise, Server ('S1') validates the message according to the
ICMPv6 Redirect message validation rules in Section 8.1 of [RFC4861],
except that the Predirect has Code=1. Server ('S1') also verifies
that Client ('A') is authorized to use the Prefix Length in the
Predirect when applied to the AERO address in the network-layer
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source address by searching for the AERO address in the neighbor
cache. If validation fails, Server ('S1') discards the Predirect;
otherwise, it copies the correct UDP Port number and IP Address for
Client ('A') into the (previously empty) TLLAO.
Server ('S1') then examines the network-layer destination address of
the Predirect to determine the next hop toward Client ('B') by
searching for the AERO address in the neighbor cache. Since Client
('B') is not one of its neighbors, Server ('S1') re-encapsulates the
Predirect and relays it via Relay ('R') by changing the link-layer
source address of the message to 'L2(S1)' and changing the link-layer
destination address to 'L2(R)'. Server ('S1') finally forwards the
re-encapsulated message to Relay ('R') without decrementing the
network-layer TTL/Hop Limit field.
When Relay ('R') receives the Predirect message from Server ('S1') it
determines that Server ('S2') is the next hop toward Client ('B') by
consulting its forwarding table. Relay ('R') then re-encapsulates
the Predirect while changing the link-layer source address to 'L2(R)'
and changing the link-layer destination address to 'L2(S2)'. Relay
('R') then relays the Predirect via Server ('S2').
When Server ('S2') receives the Predirect message from Relay ('R') it
determines that Client ('B') is a neighbor by consulting its neighbor
cache. Server ('S2') then re-encapsulates the Predirect while
changing the link-layer source address to 'L2(S2)' and changing the
link-layer destination address to 'L2(B)'. Server ('S2') then
forwards the message to Client ('B').
3.12.6. Processing Predirects and Sending Redirects
When Client ('B') receives the Predirect message, it accepts the
Predirect only if the message has a link-layer source address of one
of its Servers (e.g., L2(S2)). Client ('B') further accepts the
message only if it is willing to serve as a redirection target.
Next, Client ('B') validates the message according to the ICMPv6
Redirect message validation rules in Section 8.1 of [RFC4861], except
that it accepts the message even though Code=1 and even though the
network-layer source address is not that of it's current first-hop
router.
In the reference operational scenario, when Client ('B') receives a
valid Predirect message, it either creates or updates a neighbor
cache entry that stores the Target Address of the message as the
network-layer address of Client ('A') , stores the link-layer address
found in the TLLAO as the link-layer address(es) of Client ('A') and
stores the Prefix Length as the length to be applied to the network-
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layer address for forwarding purposes. Client ('B') then sets
AcceptTime for the neighbor cache entry to ACCEPT_TIME.
After processing the message, Client ('B') prepares a Redirect
message response as follows:
o the link-layer source address is set to 'L2(B)' (i.e., the link-
layer address of Client ('B')).
o the link-layer destination address is set to 'L2(S2)' (i.e., the
link-layer address of Server ('S2')).
o the network-layer source address is set to fe80::2001:db8:1:0
(i.e., the AERO address of Client ('B')).
o the network-layer destination address is set to fe80::2001:db8:0:0
(i.e., the AERO address of Client ('A')).
o the Type is set to 137.
o the Code is set to 0 to indicate "Redirect".
o the Prefix Length is set to the length of the prefix to be applied
to the Target Address.
o the Target Address is set to fe80::2001:db8:1:0 (i.e., the AERO
address of Client ('B')).
o the Destination Address is set to the destination address of the
originating packet that triggered the Redirection event. (If the
originating packet is an IPv4 packet, the address is constructed
in IPv4-compatible IPv6 address format).
o the message includes a TLLAO with Link ID and Preference set to
appropriate values for Client ('B')'s underlying interface, and
with UDP Port Number and IP Address set to '0'.
o the message SHOULD include a Timestamp option.
o the message includes as much of the RHO copied from the
corresponding AERO Predirect message as possible such that at
least the network-layer header is included but the size of the
message does not exceed 1280 bytes.
After Client ('B') prepares the Redirect message, it sends the
message to Server ('S2').
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When Server ('S2') receives a Redirect message from Client ('B'), it
first verifies that the requested redirection is authorized. If the
redirection is not permitted, Server ('S2') discards the message.
Otherwise, Server ('S2') validates the message according to the
ICMPv6 Redirect message validation rules in Section 8.1 of [RFC4861].
Server ('S2') also verifies that Client ('B') is authorized to use
the Prefix Length in the Redirect when applied to the AERO address in
the network-layer source address by searching for the AERO address in
the neighbor cache. If validation fails, Server ('S2') discards the
Predirect; otherwise, it copies the correct UDP Port number and IP
Address for Client ('B') into the (previously empty) TLLAO.
Server ('S2') then examines the network-layer destination address of
the Predirect to determine the next hop toward Client ('A') by
searching for the AERO address in the neighbor cache. Since Client
('A') is not one of its neighbors, Server ('S2') re-encapsulates the
Predirect and relays it via Relay ('R') by changing the link-layer
source address of the message to 'L2(S2)' and changing the link-layer
destination address to 'L2(R)'. Server ('S2') finally forwards the
re-encapsulated message to Relay ('R') without decrementing the
network-layer TTL/Hop Limit field.
When Relay ('R') receives the Predirect message from Server ('S2') it
determines that Server ('S1') is the next hop toward Client ('A') by
consulting its forwarding table. Relay ('R') then re-encapsulates
the Predirect while changing the link-layer source address to 'L2(R)'
and changing the link-layer destination address to 'L2(S1)'. Relay
('R') then relays the Predirect via Server ('S1').
When Server ('S1') receives the Predirect message from Relay ('R') it
determines that Client ('A') is a neighbor by consulting its neighbor
cache. Server ('S1') then re-encapsulates the Predirect while
changing the link-layer source address to 'L2(S1)' and changing the
link-layer destination address to 'L2(A)'. Server ('S1') then
forwards the message to Client ('A').
3.12.8. Processing Redirects
When Client ('A') receives the Redirect message, it accepts the
message only if it has a link-layer source address of one of its
Servers (e.g., ''L2(S1)'). Next, Client ('A') validates the message
according to the ICMPv6 Redirect message validation rules in
Section 8.1 of [RFC4861], except that it accepts the message even
though the network-layer source address is not that of it's current
first-hop router. Following validation, Client ('A') then processes
the message as follows.
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In the reference operational scenario, when Client ('A') receives the
Redirect message, it either creates or updates a neighbor cache entry
that stores the Target Address of the message as the network-layer
address of Client ('B'), stores the link-layer address found in the
TLLAO as the link-layer address of Client ('B') and stores the Prefix
Length as the length to be applied to the network-layer address for
forwarding purposes. Client ('A') then sets ForwardTime for the
neighbor cache entry to FORWARD_TIME.
Now, Client ('A') has a neighbor cache entry with a valid ForwardTime
value, while Client ('B') has a neighbor cache entry with a valid
AcceptTime value. Thereafter, Client ('A') may forward ordinary
network-layer data packets directly to Client ("B") without involving
any intermediate nodes, and Client ('B') can verify that the packets
came from an acceptable source. (In order for Client ('B') to
forward packets to Client ('A'), a corresponding Predirect/Redirect
message exchange is required in the reverse direction; hence, the
mechanism is asymmetric.)
3.12.9. Server-Oriented Redirection
In some environments, the Server nearest the destination Client may
need to serve as the redirection target, e.g., if direct Client-to-
Client communications are not possible. In that case, the Server
prepares the Redirect message the same as if it were the destination
Client (see: Section 3.9.6), except that it writes its own link-layer
address in the TLLAO option. The Server must then maintain a
neighbor cache entry for the redirected source Client.
3.13. Neighbor Unreachability Detection (NUD)
AERO nodes perform NUD by sending unicast NS messages to elicit
solicited NA messages from neighbors the same as described in
[RFC4861]. When an AERO node sends an NS/NA message, it MUST use its
AERO address as the IPv6 source address and the link-local address of
the neighbor as the IPv6 destination address. When an AERO node
receives an NS message or a solicited NA message, it accepts the
message if it has a neighbor cache entry for the neighbor; otherwise,
it ignores the message.
When a source Client is redirected to a target Client it SHOULD test
the direct path by sending an initial NS message to elicit a
solicited NA response. While testing the path, the source Client can
optionally continue sending packets via the Server, maintain a small
queue of packets until target reachability is confirmed, or
(optimistically) allow packets to flow directly to the target. The
source Client SHOULD thereafter continue to test the direct path to
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the target Client (see Section 7.3 of [RFC4861]) periodically in
order to keep neighbor cache entries alive.
In particular, while the source Client is actively sending packets to
the target Client it SHOULD also send NS messages separated by
RETRANS_TIMER milliseconds in order to receive solicited NA messages.
If the source Client is unable to elicit a solicited NA response from
the target Client after MAX_RETRY attempts, it SHOULD set ForwardTime
to 0 and resume sending packets via the Server which may or may not
result in a new redirection event. Otherwise, the source Client
considers the path usable and SHOULD thereafter process any link-
layer errors as a hint that the direct path to the target Client has
either failed or has become intermittent.
When a target Client receives an NS message from a source Client, it
resets AcceptTime to ACCEPT_TIME if a neighbor cache entry exists;
otherwise, it discards the NS message.
When a source Client receives a solicited NA message from a target
Client, it resets ForwardTime to FORWARD_TIME if a neighbor cache
entry exists; otherwise, it discards the NA message.
When ForwardTime for a neighbor cache entry expires, the source
Client resumes sending any subsequent packets via the Server and may
(eventually) attempt to re-initiate the AERO redirection process.
When AcceptTime for a neighbor cache entry expires, the target Client
discards any subsequent packets received directly from the source
Client. When both ForwardTime and AcceptTime for a neighbor cache
entry expire, the Client deletes the neighbor cache entry.
3.14. Mobility Management3.14.1. Announcing Link-Layer Address Changes
When a Client needs to change its link-layer address, e.g., due to a
mobility event, it performs an immediate DHCPv6 Renew/Reply via each
of its Servers using the new link-layer address as the source. The
DHCPv6 Server will re-authenticate the Client and (assuming
authentication succeeds) the DHCPv6 Renew/Reply exchange will update
each Server's neighbor cache.
Next, the Client sends unsolicited NA messages to each of its active
neighbors using the same procedures as specified in Section 7.2.6 of
[RFC4861], except that it sends the messages as unicast to each
neighbor via a Server instead of multicast. In this process, the
Client should send no more than MAX_NEIGHBOR_ADVERTISEMENT messages
separated by no less than RETRANS_TIMER seconds to each neighbor.
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With reference to Figure 4, Client ('B') sends unicast unsolicited NA
messages to Client ('A') via Server ('S2') as follows:
o the link-layer source address is set to 'L2(B)' (i.e., the link-
layer address of Client ('B')).
o the link-layer destination address is set to 'L2(S2)' (i.e., the
link-layer address of Server ('S2')).
o the network-layer source address is set to fe80::2001:db8:1:0
(i.e., the AERO address of Client ('B')).
o the network-layer destination address is set to fe80::2001:db8:0:0
(i.e., the AERO address of Client ('A')).
o the Type is set to 136.
o the Code is set to 0.
o the Solicited flag is set to 0.
o the Override flag is set to 1.
o the Target Address is set to fe80::2001:db8:1:0 (i.e., the AERO
address of Client ('B')).
o the message includes a TLLAO with Link ID and Preference set to
appropriate values for Client ('B')'s underlying interface, and
with UDP Port Number and IP Address set to '0'.
o the message SHOULD include a Timestamp option.
When Server ('S1') receives the NA message, it relays the message in
the same way as described for relaying Redirect messages in
Section 3.12.7. In particular, Server ('S1') copies the correct UDP
port number and IP address into the TLLAO, changes the link-layer
source address to its own address, changes the link-layer destination
address to the address of Relay ('R'), then forwards the NA message
via the relaying chain the same as for a Redirect.
When Client ('A') receives the NA message, it accepts the message
only if it already has a neighbor cache entry for Client ('B') then
updates the link-layer address for Client ('B') based on the address
in the TLLAO. However, Client ('A') MUST NOT update ForwardTime
since Client ('B') will not have updated AcceptTime.
Note that these unsolicited NA messages are unacknowledged; hence,
Client ('B') has no way of knowing whether Client ('A') has received
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them. If the messages are somehow lost, however, Client ('A') will
soon learn of the mobility event via the NUD procedures specified in
Section 3.13.
3.14.2. Moving to a New Server
When a Client associates with a new Server, it issues a new DHCPv6
Renew message via the new Server as the DHCPv6 relay. The new Server
then relays the message to the DHCPv6 server and processes the
resulting exchange. After the Client receives the resulting DHCPv6
Reply message, it sends an RS message to the new Server to receive a
new RA message.
When a Client disassociates with an existing Server, it sends a
"terminating RS" message to the old Server. The terminating RS
message is prepared exactly the same as for an ordinary RS message,
except that the Code field contains the value '1'. When the old
Server receives the terminating RS message, it withdraws the IP route
from the routing system and deletes the neighbor cache entry for the
Client. The old Server then returns an RA message with default
router lifetime set to 0 which the Client can use to verify that the
termination signal has been processed. The client then deletes both
the default route and the neighbor cache entry for the old Server.
The old Server SHOULD impose a small delay before deleting the
neighbor cache entry so that any packets already in the system can
still be delivered to the Client.
Clients SHOULD NOT move rapidly between Servers in order to avoid
causing unpredictable oscillations in the Server/Relay routing
system. Such oscillations could result in intermittent reachability
for the Client itself, while causing little harm to the network due
to routing protocol dampening. Examples of when a Client may change
to a different Server include a Server that has gone unreachable,
topological movements of significant distance, etc.
3.15. Encapsulation Protocol Version Considerations
A source Client may connect only to an IPvX underlying network, while
the target Client connects only to an IPvY underlying network. In
that case, the target and source Clients have no means for reaching
each other directly (since they connect to underlying networks of
different IP protocol versions) and so must ignore any redirection
messages and continue to send packets via the Server.
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Internet-Draft AERO July 20143.16. Multicast Considerations
When the underlying network does not support multicast, AERO nodes
map IPv6 link-scoped multicast addresses (including
'All_DHCP_Relay_Agents_and_Servers') to the link-layer address of a
Server.
When the underlying network supports multicast, AERO nodes use the
multicast address mapping specification found in [RFC2529] for IPv4
underlying networks and use a direct multicast mapping for IPv6
underlying networks. (In the latter case, "direct multicast mapping"
means that if the IPv6 multicast destination address of the
encapsulated packet is "M", then the IPv6 multicast destination
address of the encapsulating header is also "M".)
3.17. Operation on AERO Links Without DHCPv6 Services
When the AERO link does not provide DHCPv6 services, operation can
still be accommodated through administrative configuration of ACPs on
AERO Clients. In that case, administrative configurations of AERO
interface neighbor cache entries on both the Server and Client are
also necessary. However, this may interfere with the ability for
Clients to dynamically change to new Servers, and can expose the AERO
link to misconfigurations unless the administrative configurations
are carefully coordinated.
3.18. Operation on Server-less AERO Links
In some AERO link scenarios, there may be no Servers on the link and/
or no need for Clients to use a Server as an intermediary trust
anchor. In that case, each Client acts as a Server unto itself to
establish neighbor cache entries by performing direct Client-to-
Client Predirect/Redirect exchanges, and some other form of trust
basis must be applied so that each Client can verify that the
prospective neighbor is authorized to use its claimed ACP.
When there is no Server on the link, Clients must arrange to receive
ACPs and publish them via a secure alternate prefix delegation
authority through some means outside the scope of this document.
4. Implementation Status
An application-layer implementation is in progress.
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Internet-Draft AERO July 20145. IANA Considerations
The IANA is instructed to assign a new 2-octet Hardware Type number
"TBD1" for AERO in the "arp-parameters" registry per Section 2 of
[RFC5494]. The number is assigned from the 2-octet Unassigned range
with Hardware Type "AERO" and with this document as the reference.
The IANA is further instructed to assign a 4-octet Enterprise Number
"TBD2" for AERO in the "enterprise-numbers" registry per [RFC3315].
6. Security Considerations
AERO link security considerations are the same as for standard IPv6
Neighbor Discovery [RFC4861] except that AERO improves on some
aspects. In particular, AERO uses a trust basis between Clients and
Servers, where the Clients only engage in the AERO mechanism when it
is facilitated by a trust anchor. AERO also uses DHCPv6
authentication for Client authentication and network admission
control.
AERO links must be protected against link-layer address spoofing
attacks in which an attacker on the link pretends to be a trusted
neighbor. Links that provide link-layer securing mechanisms (e.g.,
IEEE 802.1X WLANs) and links that provide physical security (e.g.,
enterprise network wired LANs) provide a first line of defense that
is often sufficient. In other instances, additional securing
mechanisms such as Secure Neighbor Discovery (SeND) [RFC3971], IPsec
[RFC4301] or TLS [RFC5246] may be necessary.
AERO Clients MUST ensure that their connectivity is not used by
unauthorized nodes on EUNs to gain access to a protected network,
i.e., AERO Clients that act as routers MUST NOT provide routing
services for unauthorized nodes. (This concern is no different than
for ordinary hosts that receive an IP address delegation but then
"share" the address with unauthorized nodes via a NAT function.)
On some AERO links, establishment and maintenance of a direct path
between neighbors requires secured coordination such as through the
Internet Key Exchange (IKEv2) protocol [RFC5996] to establish a
security association.
7. Acknowledgements
Discussions both on IETF lists and in private exchanges helped shape
some of the concepts in this work. Individuals who contributed
insights include Mikael Abrahamsson, Fred Baker, Stewart Bryant,
Brian Carpenter, Wojciech Dec, Brian Haberman, Joel Halpern, Sascha
Hlusiak, Lee Howard, Joe Touch and Bernie Volz. Members of the IESG
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